The properties that are desirable in aqueous latex paints, namely the ability to be used at a temperature low enough for application over a long seasonal range, to withstand repeated cycles of freezing and thawing, and to form a film hard enough to avoid tackiness or blocking in the intended application, conventionally are enhanced in latex-based paint formulations by the addition of volatile coalescing solvents and freeze-thaw additives. These coalescing solvents, for example butyl carbitol acetate and 3-hydroxy-2,2,4-trimethylpentyl isobutyrate, and freeze-thaw additives, for example, propylene glycol and ethylene glycol, are volatile organic compounds (VOC) that are present in amounts up to 360 g per liter of paint (3 lbs. per gallon), not including water. With the universal recognition that VOCs are detrimental to the environment, there is a need for latex-based paints that contain no volatile coalescing solvents or freeze-thaw additives, yet which provide the requisite freeze-thaw and film-forming properties.
Latex paints employ latex binders as film formers and binders for pigments, fillers and the like, which are used in latex paints. The latex binders typically comprise emulsion polymers. Coalescing solvents normally are required because the latex binders used in latex paints must have the lowest possible film forming temperature (MFFT) and the highest possible glass transition temperature (Tg). The MFFT is the lowest temperature at which the polymer particles of the latex binder will mutually coalesce and form a continuous film when the water, which is the solvent base, evaporates. Polymers that have low MFFT extend the temperature conditions under which the paint can be applied. The Tg is the temperature at which a polymer changes from an amorphous, soft and tacky state to a glassy, hard, and rigid state. Polymers with high Tg values will result in a paint coating that will be hard, resistant to abrasion and resistant to blocking. Volatile coalescing solvents effectively lower the Tg of the polymer to meet the desired low MFFT on application, and then eventually diffuse out of the paint and evaporate under normal ambient conditions of temperature, humidity and atmospheric pressure, leaving a high Tg film. Freeze-thaw additives are added to paint formulations simply to impart freeze-thaw stability during transportation and storage.
The pigments or fillers present in the paint formulation result in anti-blocking characteristics in the paint film. The relationship between hardness of the coating and the amount of pigment is represented by pigment volume concentration (PVC), which is the fractional volume of pigment in a unit volume of resin. Thus, low PVC coatings, such as semi-gloss paints, contain relatively low levels of pigment, and high PVC coating compositions, such as satin to flat paints, contain high levels of pigments. Polymers with low Tg and MFFT in low PVC paint formula will exhibit blocking tendencies. On the other hand, the soft latices will show anti-blocking characteristics in high PVC paint formulas. In low PVC paint formulas, glass transition temperature of the polymer (Tg) determines the hardness of the coating. In high PVC paint formulas, pigments determine the hardness of the coating. The Tg of the polymer can be calculated using the Fox equation. 1/Tg (polymer)=W(a)/Tg(a)+W(b/Tg(b) where W(a) and W(b) are the weight fractions of comonomers (a) and (b) and Tg(a) and Tg(b) are the glass transition temperatures for homopolymers (a) and (b), respectively in .degree.K. Glass transition temperatures for various homopolymers are available in may literature sources, including J. Brandup and E. H. Immergut, Polymer Handbook, 2nd ed., John Wiley & Sons, New York, pp. 139-192 (1975).
There is a growing concern about the potentially adverse environmental and health effects of many of the volatile coalescing solvents and freeze-thaw additives. There is a growing need for polymers, for use in latex binders in latex paints, which will provide desired hardness properties, adequate film formation at low temperature, and flexibility. In addition, it is also desirable to eliminate volatile coalescents and freeze-thaw additives from trade sale paints without compromising physical properties such as coating hardness, low MFFT and freeze-thaw stability. Accordingly, it would be desirable to develop polymeric latex binders which have the balance of MFFT and Tg required for use in latex paint compositions, which are free of volatile coalescing solvents or freeze-thaw additives and which maintain adequate freeze-thaw stability and abrasion resistance.
Latex binders based on ethylene/vinyl acetate (EVA) copolymers may be used in the formulation of latex paints. The EVA copolymers are known to provide latex paints with film-forming properties and abrasion resistance which are sufficient for their intended use. However, these polymers have been found not to provide freeze-thaw stability to formulated latex paints which utilize latex binders based on EVA. This has been found to be true even with the addition of significant amounts of volatile freeze-thaw additives to the latex paints. This is unlike conventional latex paints based on acrylic and vinyl/acrylic latex binders, where volatile freeze-thaw additives are used to provide freeze-thaw stability to the latex paints.
It would be desirable to develop EVA-based latex binders which can be used to prepare latex paints, which not only exhibit the desired film-forming and abrasion resistance properties for which EVA is known, but also which are freeze-thaw stable in the absence of volatile coalescing or freeze-thaw solvents.
It now has been discovered that EVA-based latex binders according to the present invention may be used to formulate EVA-based latex paints which not only are freeze-thaw stable in the absence of volatile coalescing or freeze-thaw solvents, but which retain the film-forming properties and abrasion resistance for which EVA is known.